Well, I called Optus' complaint line (133 937) and I must say the call went better than expected. Almost too good to be true, in fact.
The agent I spoke to assured me that the email that was sent out was a "generic" email and that "the system will check all your inclusions and apply those inclusions to your new plan" (his exact words).
I (twice) then asked him to clarify: so Global Roaming will be included on my new plan? "That's correct".
So I should ignore what the email says about how "international roaming will no longer be included on your plan"? "Yes, you should ignore what the email says as it was sent out to all affected customers and doesn't affect your plan".
Still somewhat incredulous, I asked whether he would be able to put what he'd told me in writing. "I will add a note of this conversation to your account".
I asked: and that note will specifically include that Global Roaming will be included on my new plan after August 3? "Yes, it will".
I'm still a little sceptical and would like to know how other people go. And of course I'll still be double-checking and triple-checking that international roaming will be included on my new plan, and obviously taking it further if it isn't. But so far so good...